This invention relates to digital FM radios and, more particularly, to digital FM demodulators useful in a digital FM radio.
A digital FM radio typically includes an FM demodulator that outputs a signal for further baseband processing prior to being converted to an analog signal for driving speakers. A typical perspective with regard to the signals that are received by the FM demodulator is that there are two signals, I and Q, that represent the complex baseband FM modulated signal. These two signals are generally considered to be represented by A(t)ejxcex8(t) where A(t) is the amplitude and xcex8(t) is the phase angle of the FM modulated signal. The output of the FM demodulator is commonly called MPX, which is ideally dxcex8/dt, but in actuality is typically a distorted version of dxcex8/dt.
This distortion is manifested by a non-linear frequency response that results in sub-optimal FM stereo separation and some inter-symbol interference (ISI) in the FM digital sub-carriers such as radio data system (RDS) symbols. The RDS symbols are used for indicating traffic information and FM station identification information. Thus, the quality of the radio is directly affected by the degree to which the MPX signal is an accurate representation of dxcex8/dt.
It is also desirable to achieve as much quality as possible in cases where the radio is distant from the source. A typical use of a radio is in automobiles so that it is common in such an application for the radio to move in and out of the range of a radio station""s transmitter. Thus, it is desirable for radio to have both wide dynamic range and an accurate dxcex8/dt, which has been difficult to achieve.
Thus, there is a need for a radio with an FM demodulator that provides for highly accurate dxcex8/dt and for wide dynamic range.